Photopolymerizable compositions and films containing binder, monomer, initiator and chain transfer agent are described in the prior art and sold commercially. One important application of photopolymerizable layers is in graphic arts. A need exists in the graphic arts field to render faithful proofs which describe the image quality that can be attained prior to the printing process. Specifically, it is desirable to demonstrate the appearance and the quality of the printed product prior to its production. The actual mounting of printing plates on a printing press is expensive and time consuming. Adjustments in the printing plate are sometimes necessary in order to achieve the right tonal range, etc. In other cases, it is necessary to remake the plate, if there are any defects in it, such as may be caused by improper exposure of a color separation negative from which a plate is generated.
A number of proofing processes are commercially available. Several of these are capable of giving separate films containing colored images, which on superimposition give a multicolored image that approximates the ultimate pattern generated on the printing press. Other processes depend on selectively toning layers of partially exposed surfaces, to give surprints which more closely resemble the images that are generated on printing than the overlay films described earlier. These processes, however, do not result in the most desirable proof, i.e., one which gives a surprint that is indeed a printed image on unmodified paper stock as is used in printing. Furthermore, the previously cited methods do not permit the facile formation of multiple prints as are frequently required in the printing industry, as for example, when the proof is employed as a press guide in two different locations. The technology described herein addresses the need to make multiple surprints and to overcome the limitations of several commercial proofing processes.
Photopolymerizable layers are currently being used as electrostatic masters for analog color proofing. For this application, a photopolymerizable or photohardenable layer is coated on an electrically conductive substrate and contact exposed with an ultraviolet (UV) source through a half-tone color separation negative. The photopolymerizable composition hardens in the areas exposed with an ultraviolet source due to polymerization and remains in a softer state elsewhere. The differences between the exposed and unexposed areas are apparent in the transport properties, i.e., the unexposed nonpolymerized areas conduct electrostatic charge while the UV exposed areas are substantially non-conductive. By subjecting the exposed photopolymerizable layer to a corona discharge a latent electrostatic image is obtained consisting of electrostatic charge remaining only in nonconducting or exposed areas of the photopolymerizable layer. This charged latent image can be developed by application of a liquid or dry electrostatic developer thereto. When the developer has a charge opposite to that of the corona charge, the developer selectively adheres to the exposed or polymerized areas of the photopolymerizable layer. It is desirable to permit selective toner deposition on the imagewise exposed and charged photopolymerizable layer within a short time after charging. That is, there is the need for a more rapid decay of the unexposed (background) areas of the photopolymerizable or photohardenable layer. As long as a significant amount of charge resides on the unexposed (background) areas, developer will be deposited on these areas, therefore requiring a longer time period between charging and applying developer if background coloring is to be avoided. Although single color electrophotography is a reliable mature technique, color on color electrophotography is relatively new and the application of four different color developer layers on top of each other has its own problems.
While slow charge decay is a problem, we consider the most serious problem in the preparation of color proofs using electrostatic systems to be backtransfer. It was discovered that when a second color developer was transferred from the photohardenable master on top of an existing image on paper, the developer layer originally on the paper partially backtransferred to the electrostatic master during the second transfer. The backtransfer problem worsens when dealing with four layers of developers, since in that case all the previously transferred colors can partially backtransfer from the paper onto the surface of the master. Therefore, the final image on paper is unacceptable due to its degraded color and resolution. In attempting to deal with the backtransfer problem we noted, for example, that the negatively charged toner particles in the liquid electrostatic developer when backtransferred surprisingly were found to be neutral or have positive charges. This charge reversal or neutralization suggested that the large transfer fields partially electrolyzed the toner particles. Charge reversal also implied that toner particles will backtransfer since an electric field that drives negative particles towards the paper would drive positive particles towards the master.
Furthermore, we learned that the toner neutralization occurred on the paper and at the photopolymer electrodes. Backtransfer could be overcome by blocking the toner neutralization either by using dielectric coated paper or by washing the photopolymerizable layer surface with a solution of charge director and carrier liquid with conductivities above a determined threshold value. These approaches, however, are not practical as it is undesirable to use non-standard papers and to wash the surface of the photopolymerizable layer.
Backtransfer has not been observed when the charged surface is a selenium photoconductor and is not a serious problem on silver halide masters. Charged photopolymerizable layers are different with respect to backtransfer. For example, up to 80% of a toned image can be backtransferred to a photopolymerizable master under high ambient humidities and high transfer field conditions. It is therefore believed that the resistivity of the transfer zone and the nature of the charge carrier play important roles in developer backtransfer. In an attempt to overcome the disadvantage of backtransfer, the photopolymerizable composition was formulated to include additives that modified the electrochemistry at the surface of the photopolymerizable layer so that the particular liquid electrostatic developer would transfer from the master onto the paper or subsequent transferred image layer without electrically modifying the toner particles in the developer.
It has now been found that charge decay of the unexposed areas of a photopolymerizable or photohardenable layer and backtransfer of previously developed and transferred images to the surface of the photohardenable layer of an electrostatic master can be greatly improved by introducing into the photohardenable composition used to form the layer an acidic additive of the type described below.